Pressure sensors are widely used in applications ranging from commercial and tactical aircraft to aerospace and automotive vehicles. Pressure sensors are typically incorporated into transducers for use in various control systems for aircraft and vehicles, particularly flight and servo controls, engine and fuel controls, thrust vectoring, and force feedback. Because of these varied applications, the pressure sensors must operate under adverse conditions such as temperature and pressure extremes, vibration, and so on.
Capacitors in general consist of a pair of oppositely-charged conductive plates in a spaced relationship. The medium occupying the area between the plates is called the dielectric, which is generally a nonconductive material such as ceramic or simply air. The value or capacitance of the capacitor depends upon several variables, including the type of dielectric, the surface area of the plates, and the distance between the plates. Therefore, if any of these variables change, the capacitance of the capacitor changes accordingly. Generally speaking, a capacitive sensor is constructed so that the area of the plates does not change, and the medium between the plates (the dielectric) does not change; only the distance between the plates changes. As capacitance is inversely proportional to the distance between the plates, capacitance increases as distance decreases.
In order to measure a change in pressure with a capacitive sensor, at least one of the plates is flexible or deflectable, such that if there is a change in pressure, the force on the outside of the deflectable plate increases or decreases, thereby deflecting the plate and therefore changing the distance between the plates. Accordingly, capacitance changes.
To determine the pressure change, capacitance is continuously measured in the sensor. If the capacitance increases, the change in distance which caused the increase in capacitance is calculated based on known values. Furthermore, the increase in force required to cause the plate to flex the calculated distance is then determined, from which the magnitude of the pressure increase is determined. A capacitive sensor incorporating this principle has been disclosed by U.S. Pat. No. 4,425,799, which was granted on Jun. 14, 1983, and is assigned to the assignee of the present invention.
In order for the capacitive pressure sensor to reliably operate in certain critical applications, the chamber between the plates is often evacuated. If air were present within the chamber and the sensor were to operate at high or low temperatures (e.g., 150 degrees celsius or -40 degrees celsius), the air would accordingly expand or contract, thereby exerting an internal force on the deflectable plate and lessening the accuracy of the sensor. Accordingly, the chamber must be hermetically sealed to ensure that the vacuum within the chamber is maintained over time. Evacuating the chamber between the plates also yields more favorable capacitance values for the sensor.
Methods are present in the related art for hermetically sealing evacuation passages formed in capacitive sensors. These methods typically entail numerous steps, including metallizing the surfaces, firing with thick films such as palladium or gold, adding flux, soldering, evacuating, and heating for a length of time to ensure that a substantially hermetic bond has formed. This process is not only time consuming and expensive but also demands highly-specialized manufacturing conditions in order to maintain quality control standards.
Therefore, there is a need in the art for a capacitive pressure sensor with a relatively easily-formed hermetic seal. Furthermore, there is a need for a method for sealing an evacuation passage in a capacitive pressure sensor in an easy, reliable, and cost-effective manner.